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`Mercedes-Benz USA, LLC, Petitioner - Ex. 1011
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`DESIGN PERSPECTIVE
`
`LEDs for exterior lighting
`
`Until recently international regulations have mandated
`against the use of Light Emitting Diodes for exterior light-
`ing on vehicles. A terminology change recently ratified
`would appear to remove this barrier. Does this now mean
`that LEDs will become as commonplace on the outside of
`vehicles as they currently are on the Inside? Hewlett-
`Packard is a leading supplier of optoelectronic devices
`and Martin Lister of the company’s Components Group
`reviews the developments made in LED technology and
`their applications in automotive engineering.
`
`In the 1960's production processes
`were developed that allowed a p-n junc-
`tion, traditionally only able to emit non-
`vlsble Infra-red light, to emit light in the
`red area of the visible spectrum. This
`development led Hewlett-Packard to
`introduce. in 1968. the first commercially
`available Light Emitting Diode (LED)
`display and so launch the era of a com-
`pletely new generation of lighting prod-
`ucts. it seems that ever since the LED
`was first introduced there has been
`speculation on if and when it would be
`used in automotive lighting applications.
`Within the driver's compartment devel-
`opments have certainly been made,
`however, the use of LEDs for exterior
`applications has been less prevalent.
`This is not primarily due to the lack of
`technological development in LEDs as
`high-brightness devices now exist in a
`number of different colour combinations
`whose performance clearly lit them for
`exterior lighting applications. The bigger
`challenge to the take-up of LEDs has
`been regulatory. The lighting on vehi-
`cles ls governed by a large body of
`international legislation. Up until recently
`this legislation has been specific in its
`recommendations tor exterior lighting; it
`has unequivocally specified that incan-
`descent lamps should be used. with this
`terminology in place it seemed that
`despite the technological developments
`taking place there would never be an
`opportunity for the use of LEDs on the
`outside of vehicles.
`However. over the past 12 months
`there has been a change to the termi-
`nology used In this legislation that
`appears to have removed the regulatory
`barriers. In amendments to the legisla-
`tion. the terminology has been changed
`from ‘incandescent lamp‘ to ‘light
`source‘. This now means that lighting
`devices other than filament bulbs can be
`used in exterior applications and this
`obviously includes devices such as
`LEDs. So it the regulatory barriers have
`
`now been removed does this mean
`there will be a rush to design in solid
`state lighting on all new cars’? To
`answer this question it is perhaps first
`appropriate to review the benefits to the
`vehicle designer of using these devices.
`The first and overwhelming advan-
`tage ot course is that of reliability. LEDs
`generate their light output from a solid
`state device as opposed to a white hot
`glowing filament. intrinsically this implies
`greater reliability and this is borne out by
`the data. LEDs are typically rated for
`over 50 000 hours of operating life com-
`pared to a few thousand at best for
`incandescent lamps. in addition cars
`and trucks provide a very extreme envi-
`ronment lor any electrical device.
`Resistance to vibration and shock and
`extremes of temperature and humidity
`are essential for good reliability. The
`solid state LED easily outperforms the
`incandescent lamp with its fragile fila-
`ment in this environment.
`One of the benefits to the vehicle
`manufacturer that result from this relia-
`bility advantage is greater freedom in
`styling. LED lighting-panels can be
`located on areas subject to shock and
`vibration where traditionally incandes-
`cent iights would not be appropriate.
`
`Fig 1. Critical reflectance angles for cervical
`straight walled reflector cavity
`
`-A
`The reliability of the devices‘ V:
`that they can be incorporated ‘
`vehicle on a sealed-for-lite -~
`.
`designers do not have to
`«
`:-
`sion for access to the device‘ --
`ing. It can thus be integrated T
`body panels making for as 7
`more aesthetically-pleasing - -
`,
`since LED lamps for vehicles,
`pie light sources designers can ,
`ment with lamp appearances‘ »
`crete ‘points’ of light to evenlyl
`for cosmetic lights or appiiques.
`01 a direct part-lor-parl » '
`a LED light source is higher - --
`its incandescent counterpart.
`-
`price is not necessarily the best
`for comparison. LEDs promote ~-
`ings in other areas. This rell
`reduces design complexity. .
`reduces sheet metal, additional 4
`pans. labour and other costs.
`:
`significantly less power. up to one
`oi that used by incandescent -
`This gives the potential for lower 2
`lower weight electrical systems. 7
`generate lower levels of heat
`incandescent lamps, ellminati
`need for specialised high tern .-
`resistant epoxies in the lamp -
`Finally. of course, a sealed for life
`ing system can significantly reduce
`ranty costs and improve customer -.
`faction.
`There is one other advantage
`appears to be given by the use of
`in exterior lighting and this is .-
`not necessarily expected orlgl =
`their manufacturers‘. they turn on -
`er than incandescent lamps. Testi
`shown that when used in a stop
`they give an earlier warning to
`f
`vers. The difference is between I ;
`200 ms. At motorway speeds
`equates to a r's length in stqtpig
`tance. This can contribute dir
`safety and has the potential to :.'f
`the probability and severity of rear
`collisions.
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`f\..o—-L -u-Iklps.-on-ulna
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`Page 2 of 3
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`
`Recent technology advances
`Light is emitted from an Light Emitting
`Diode due to the recombination of elec-
`trons and holes inside the p—n junction.
`Each recombination results in the
`release of a photon of light. The materi-
`als whose properties are well suited to
`exhibiting this phenomenon are those
`found in the third and fifth columns oi
`the Periodic Table. the Ill-V materials.
`Not all of the light released is seen by
`the observer. losses occur, typically In
`three areas. The first of these is loss
`due to absorption within the LED materi-
`al. The photons released are emitted in
`all directions. if the substrate on which
`the junction is grown is opaque (as with
`Gallium Arsenide) only those photons
`emitted upwards and within a critical
`angle can be utilised as light output.
`The second cause oi loss is referred
`to as Fresnel loss. Fresnel loss is intro-
`duced ln the packaging of the LED.
`it
`occurs when light is reflected back at
`the interface of two materials whose
`index of refraction diiier. The third cause
`of loss is Critical Angle loss. The effect
`of this is seen when an observer moves
`in relation to the LED. when the observ-
`er moves oil the axis oi the device the
`light output decreases markedly. Since
`the phenomena that result in those less-
`es cannot be overcome LED develop-
`ment has focused on making advances
`in the production of the lil-V materials
`and on the packaging used. For exam-
`ple. Hewlett-Packard has developed
`devices using Aluminium Gallium
`Arsenide (AlGaAs) in which the sub-
`strate ls etched away to eliminate pho-
`ton absorption intemally. The result is a
`Transparent Substrate device designat-
`ed TS-AlGaAs. Using transparent sub-
`strate technology HP has produced
`LEDs that give a light intensity 100
`times brighter than the traditional LED.
`Also. beiztuse the optical flux emitted
`by a LED is fairly low. LED manufactur-
`ers in the past have introduced multiple
`LED die in a single package. Devices
`with 2 die have been common and
`some years ago HP introduced a device
`with 4 die. Thesedevlces produced suf-
`ficient optical flux to be used in high
`brightness applications but unfortunately
`their cost mandated against them. An
`alternative approach used by the com-
`panytopaokagethesingiedieisaspin
`Dli?. Four of the pins are then used to
`conduct heat away irorn the cathode of
`the device allowing the lamp to operate
`atuptoioomaofdrivecurrentwithout
`exceeding its power rating. in these
`series of Brewster lamps the LED die is
`mounted in an optical reflector providing
`improved optical periormance over con-
`ventional packaging techniques. The
`lamps useadeeper reflectorcIshallow-
`ing them to catch the light emitted from
`the sides of the diode and direct it
`upwards. where a convex lens further
`at
`
`Fig 2. ‘Sweeping’ turn-signal indicator pro-
`posal by Stanley Electric rvith. below, their
`proposal for a combined LED rear lamp and
`proximity sensor
`collimates the light. The resulting optical
`output and radiation pattern provides
`significant improvements over conven-
`tional indicators using the technology.
`The use oi LEDs in high brightness
`applications requires good optical
`design principles to be iollowed in the
`design oi the housing surrounding the
`lamps. The housing is more critical for
`diodes than for incandescent bulbs for a
`number of reasons. Firstly the lower
`optical output of these devices requires
`that none of this be wasted. care is
`need to ensure that the maximum
`amount of light from the unit strikes the
`legend area. Secondly incandescent
`lamps tend to have a radiation pattern in
`which light is emitted equally in all direc-
`tions whereas. as noted above. the
`luminous intensity tends to peak in one
`direction.
`A properly designed white reflective
`optical cavity can utilise most of the opti-
`cal ilux emitted by a light source. the
`cavity redirects the light emitted at wide
`angles so that it strikes the rear oi the
`legend. The performance oi the cavity
`depends upon its geometry and equa-
`tions can be derived to calculate the
`amount of light output exiting the hous-
`lng after reilection from the cavity walls.
`For any optical cavity critical angles
`exist and these are shown in Fig 1 for a
`conical straight walled cavity. At angles
`less than 0,, light rays emitted from the
`light source will not interact with the cav-
`ity at all. Some rays will strike the cavity
`wall and reflect at zero degrees; this
`angle 0; is twice the cavity wall angle.
`0... At one angle 0, light rays will strike
`one cavity wall and graze the top oi the
`other side of the cavity. Sonia fraction of
`the light will be trapped by the cavity. for
`angles greater than Ob“. light will be
`reflected downwards. Finally some light.
`at angles greater than am, will miss the
`entrance to the cavity altogether.
`HewIett—Packard has conducted
`experiments with a number of cavity
`geometries including round and square
`
`conical. As a result of these“:
`merits a cavity was developed
`'
`a high mount brake light
`comprises awhite.r -4
`
`- "
`
`~
`
`‘
`
`A
`
`’
`
`dioxide filed polycarbonate,
`had straight walls with a 19
`angle. The exit aperture was
`15 mm and the entrance
`square. The perionnance of
`was characterised using a
`LED lamps each measured
`of drive current. For all lam"
`sured. luminous intensities in
`1000 candela/mi’ were recorded
`and centre with 300 to 400
`COITIGIS.
`
`'
`‘..
`
`The combination of de ~ --
`the materials. die packagintl. anti
`.,_
`oi the optical cavity together
`LEDs can now provide the -=
`required for exterior lighting
`tions. Colour potential has"
`changed; originally LEDs only _ A’
`light in the red area oi the :-..
`Developments made in the i _ =
`have meant that today devl: .
`available in red, high efficiency
`orange. amber. yellow and ‘
`Further developments being
`promise the availability oi blue ‘_.
`giving the potential, when
`-:-‘
`with the other primary colours. '-fif
`state indicator that emits whla '- ‘
`
`Exterior applications
`Virtually all of the maior vehicle
`faciurers now have projects
`-
`A
`today that are either investl
`-‘:-
`developing exterior lighting .-
`based on LEDs. One of the iirstd_
`is tor high-mount stop lights:
`size, resistance to shock and lull; »
`1-
`output (reducing nuisance to tea?-
`1
`sengers) favouring their use
`convertible cars there is no rear;
`screen in which to mount the
`resistance of diode lamps to
`vibration means that they can be N"
`ed on the boot lid and several
`_A "
`turers, for example, are currently _
`oping lights incorporated into then
`spoiler.
`‘
`Another application being a‘
`investigated is for side markers‘
`heavy goods vehicles. a harsh :
`-
`ment for lighting products. As :- =
`lite units LEDs outlast incand
`lights, keeping vehicles on the i -
`related application is for direction ‘'A -
`tor repeaters on cars. Ford in the
`has recently announced their ~ v«‘_.
`to use LEDs on the new Thu - _
`model: in this case for cosmetic - _'
`as, as applique lighting made --
`through the use oi the low proiile. Y
`vides an example of how the Lise
`ldstatedevicescanexpandthe
`tions oi lighting on vehicles anti’ p
`LEDsneednotbeviewedas
`iorincandescentlights. Enter i69= *
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`October/Noveiniié'i"‘: V
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`Page 3 of 3